1. Field of the Invention
The present invention relates to a method of making a silicon semiconductor film having crystalline performance, for example, a polycrystal silicon film, a single crystal silicon film or a microcrystal silicon film. A crystalline silicon film fabricated by using the present invention is used in various semiconductor devices.
2. Description of the Related Art
There has been known a thin film transistor (hereinafter, referred to as TFT or the like) using a thin film semiconductor. TFT is constituted by forming a thin film semiconductor, particularly a silicon semiconductor film on a substrate and by using the thin film semiconductor. Although TFT is utilized in various integrated circuits, TFT attracts attention particularly as a switching element provided to each pixel of an active matrix type liquid crystal display device and a driver element formed at a peripheral circuit portion thereof. Further, TFT attracts attention as a technology indispensable also in a multiple structure integrated circuit (three-dimensional IC).
Although it is convenient to use an amorphous silicon film for a silicon film utilized in TFT, electric properties thereof are far lower than those of a single crystal semiconductor film used in a semiconductor integrated circuit. Accordingly, TFT has been used only in a limited usage such as a switching element of an active matrix circuit. A silicon thin film having crystalline performance may be utilized for promoting the properties of TFT.
A silicon film having crystalline performance is referred to as a polycrystal silicon film, a polysilicon film, a microcrystal silicon film or the like other than a single crystal silicon film. To provide such a silicon film having crystalline performance, an amorphous silicon film is firstly formed and thereafter, the film is crystallized by heating (thermally annealing) the film. The process is referred to as solid phase growth process since an amorphous state is transformed into a crystalline state while maintaining a solid state.
However, in the solid state growth of silicon, a heating temperature of 600° C. or higher and a time period of 10 hours or more are needed and an inexpensive glass substrate is difficult to use as a substrate. For example, Corning 7059 glass used in an active type liquid crystal display device is provided with the-strain point of glass of 593° C. which is problematic in performing thermal annealing at 600° C. or higher when large area formation of the substrate is considered.
According to a research conducted by the inventors on such a problem, it has been found that when a certain kind of metal element of nickel or the like is piled up on the surface of an amorphous silicon film by a small amount and thereafter the film is heated, the crystallization can be performed at 550° C. for a processing time period of about 4 hours. Naturally, a silicon film having a further excellent crystalline performance can be provided by performing the annealing process at 600° C. for 4 hours. (refer to Japanese Unexamined Patent Publication No. JP-A-6-244103).
In order to introduce such a small amount of metal element, there are a method of piling up a film of a catalyst element or a compound thereof by a sputtering process (Japanese Unexamined Patent Publication No. JP-A-6-244104),a method of forming a film of a metal element or a compound thereof by means of spin coating (Japanese Unexamined Patent Publication No. JP-A-7-130652), a method of forming a film by decomposing a gas including a metal element by thermal decomposition, plasma decomposition or the like (Japanese Unexamined Patent Publication No JP-A-7-335548) and the like which may be used according to the respective characteristic.
Further, it is possible to introduce a metal element selectively at a specific portion and to expand crystal growth from a portion where the metal element is introduced to surroundings by heating (lateral growth process or horizontal growth process). Crystal silicon provided by such a method has an orientation of crystallization and therefore, extremely excellent properties are shown in accordance with the orientation.
Further, it is also effective to further improve crystalline performance by irradiating strong beam such as laser beam or the like after a crystallizing step using a metal element (Japanese Unexamined Patent Publication No. JP-A-7-307286). Further, according to the above-described lateral growth process, it is effective to perform thermal oxidation in succession thereto (Japanese Unexamined Patent Publication No. JP-A-7-66425).
When crystallization is performed by using a metal element in this way, a crystalline silicon film having excellent quality is provided at a lower temperature in a shorter period of time. A temperature of heating treatment is preferably falls in a range of 450 through 650° C., particularly preferably, in a range of 550 through 600° C., although strongly dependent on the kind of an amorphous silicon film.
However, the most serious problem in the method is removal of metal element. It cannot be disregarded that a metal element introduced in a silicon film effects adverse influence on the electric properties and reliability. Particularly, in the step of crystallization using a metal element, as the mechanism of step, the metal element remains in the film mainly as a conductive silicide which constitutes a significant factor of defect.
It is generally known that a metal element (particularly, nickel, palladium, platinum, copper, silver or gold) can be removed by a heating treatment in an atmosphere of hydrogen chloride at high temperatures. However, a high temperature treatment at about 1000° C. is needed therefor which is against the thought of low temperature process using a metal element. The present invention has been carried out in view of the above-described problem and it is an object of the present invention to provide a method effective in removing a metal element by providing conditions preferable for gettering.